From the FT. With the payout will also come a hefty tax bill for the company, but tax rules will mean the company will actually receive a $500 million credit in 2013!!

So it is no surprise that the personal wealth about to be showered on its army of 20-something engineers and other employees will outshine anything seen since the days of the dotcom bubble.

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The financial implications will also have unexpected effects on Facebook’s tax bill and its accounting after it becomes a public company. Among the most surprising: a $500m US tax refund to the company early next year. It is a payment that is likely to be controversial given the state of the US fiscal position and Facebook’s own conspicuous success.

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At an estimated $23bn, the 3,000-odd employees stand to collect an average of more than $7m each. Those gains, though, will be very unequally shared, with it will be the first employees taking the bulk of the money. Young start-ups greatly reduce stock awards for new hires once their success seems assured – as Google did in 2002.

“It will create all sorts of internal equity problems for Facebook,” said Kevin Murphy, professor of finance at the USC Marshall School of Business. The “first-mover advantage” of the workers who came in first will make them a privileged – and very wealthy – class inside the company, he added.

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In tax terms, the company will pay some $5bn to cover its employees’ personal tax bills six months after its IPO, on top of the $1.5bn-$2bn tax liability that Mr Zuckerberg himself is expected to face. That is based on the 224m restricted stock units that will vest in November, along with a portion of a remaining block of 102m that has yet to vest.

Under US tax rules, Facebook will then be able to offset those amounts against its own profits – effectively erasing its own US tax bill this year.

Also, since the company can carry these “losses” back two years, Washington will actually give Facebook a $500m refund in 2013, according to the company.

Given the scale of the employee taxes already expected this year, as well as taxes on future profits from other stock benefits, that makes it unlikely that Facebook will pay any US taxes for some time.

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Not all Facebook employees will share in the wealth. Some early employees, who were handed options, ended up with nothing: faced with paying tax on any options they exercised when they left the company, some let their options lapse rather than face a bill on shares whose value was uncertain.

In all, some 427m options have already been exercised, according to Facebook’s regulatory filings. Those shares would be worth an additional $19bn at today’s notional private market value, though many were sold early by departing employees, taking advantage of the new private markets that have sprung up for young tech start-ups.

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How many will take the money and run will now become a question uppermost in the minds of Facebook’s top executives, as well as the many other Silicon Valley companies hungry to hire talented staff. Much will depend on whether the company can convince its workers that it will remain one of the Valley’s coolest places to work, and whether its value continues to rise.